1. Field of the Invention
The present invention relates to a developer used for an image forming method (e.g., an electrophotographic method) which visualizes an electrostatic charge image and for a toner jet method.
2. Description of the Related Art
Conventionally, various methods such as an electrostatic recording method, a magnetic recording method, and a toner jet method are known as image forming methods. For example, the electrophotographic method generally includes: forming an electrical latent image on an image bearing member such as a photosensitive member utilizing a photoconductive material by various means; developing the latent image by use of a toner to form a toner image which is a visible image; optionally transferring the toner image to a recording medium such as paper; and fixing the toner image on the recording medium by means of heat, pressure, or the like, so as to obtain an image.
In such a method, an image forming method has been generally used, which includes: cleaning a residual toner which was not successfully transferred to the recording medium and remains on the image bearing member by various methods to collect the residual toner as a waste toner in a waste toner container, and repeating the above-mentioned image forming process.
On the other hand, a technique referred to as a cleaning coinciding with developing or a cleanerless image forming method has been proposed as a system which does not produce a waste toner. For example, a technique relating to the cleanerless image forming method is disclosed in JP 05-053482 A and JP 05-061383 A. However, such publications do not describe a detailed and specific configuration of the entire system.
The cleaning coinciding with developing which inherently has no cleaning apparatus necessarily requires rubbing the surface of an image bearing member with a toner or a toner carrier. Therefore, various contact developing methods in which a toner or a toner carrier is in contact with an image bearing member have been studied as a preferably applicable developing method. This is because a configuration in which a toner or a toner carrier is in contact with and is rubbing an image bearing member is conceived as advantageous in order to collect a transfer residual toner in developing means. However, in a cleaning coinciding with developing or a cleanerless process employing the contact developing system, deterioration of a toner due to long-term use, or deterioration or abrasion of the surface of a toner carrier or an image bearing member is caused, in other words, sufficiently satisfactory solutions with regard to durability have not yet been made.
In contrast, a direct injection-charging system capable of performing a cleaning coinciding with developing not only in a contact developing system but also in a non-contact developing system and producing no active ion such as ozone has been proposed as a novel and environmentally preferable technique. Brief description will be made with regard to the direct injection-charging system.
The direct-injection electrification system is a system in which charge is directly injected from a contact charging member to a member to be charged, thereby charging the surface of the member to be charged. This system is also referred to as a direct charging, an injection charging or a charge injection-charging. More specifically, in this system the contact charging member having medium resistivity comes in contact with the surface of the member to be charged, thereby directly injecting charge to the surface of the member to be charged without causing a discharge phenomenon (i.e., basically not using a discharge). Therefore, even if a voltage applied to the contact charging member is equal to or less than a discharge threshold, it is possible to charge the member to be charged at a potential corresponding to the applied voltage. Since such a charging system does not produce an ion, there arises no problem due to a discharge product. Attention should be paid that in such a direct injection-charging, the number of contact points and a contact area between the contact charging member and the member to be charged drastically contribute to chargeability. Therefore, a configuration in which the contact charging member is provided with more close contact point or in which a contact time of the contact charging member with the member to be charged is maintained longer is required in order to achieve contact of the contact charging member with the member to be charged for a longer period of time.
As means that improves density of a contact point between a charging member and a member to be charged, a method has been proposed in which a charge accelerating particle having conductivity at an abutting portion between the charging member and the member to be charged (specifically, an image bearing member) is made to intervene so as to ensure injection chargeability with high efficiency.
For example, JP 10-307456 A discloses an image forming apparatus in which a developer containing a toner particle and a conductive charge accelerating particle having a particle diameter of one-half or less of a toner particle diameter is applied to an image forming method employing an cleaning coinciding with developing and a direct-injection electrification system. According to this proposal, it is possible to obtain an image forming apparatus employing a cleaning coinciding with developing, which is capable of significantly reducing an amount of a waste toner without producing a discharge product and which is low cost and advantageous for miniaturization of an apparatus, thereby it is possible to obtain an excellent image without causing deterioration in chargeability, or causing light shielding or diffusion of image exposure.
Furthermore, JP 10-307456 A discloses an image forming apparatus employing a cleaning coinciding with developing in which a conductive fine particle is externally added to a toner. According to this proposal, the conductive fine particle existing at least at an abutting portion between a flexible contact charging member and an image bearing member is attached to the image bearing member in a developing process, remains on the image bearing member even after a transferring process, and is carried and intervened thereon. As a result, it is possible to obtain an excellent image without causing deterioration in chargeability or causing light shielding of image exposure.
Furthermore, JP 10-307421 A discloses an image forming apparatus having a transfer promoting effect due to a conductive fine particle. According to this proposal, a developer containing a conductive fine particle having a particle diameter of 1/50 to ½ of a toner particle diameter is applied to an image forming method employing a cleaning coinciding with developing and a direct injection-charging system.
In addition, JP 10-307455 A describes that a particle diameter of a conductive fine particle is set to be equal to or less than a dimension of one pixel, and that a particle diameter of a conductive fine particle is set to be 10 nm to 50 μm to obtain more satisfactory charge unevenness.
JP 10-307457 A describes that a particle diameter of a conductive fine particle is set to be approximately 5 μm or less and preferably 20 nm to 5 μm to make effects of a charge deteriorated portion on an image hardly visually recognizable in consideration of human visual characteristics.
In addition, JP 10-307458 A describes an image forming method employing a cleaning coinciding with developing and a direct-injection electrification system. According to this disclosure, it is possible to avoid disturbance of development with a toner upon developing or leakage of a developing bias through a conductive fine particle by setting a particle diameter of the conductive fine particle to be equal to or less than a toner particle diameter, thereby obtaining an image having no deterioration. It is also possible to solve a problem in that a conductive fine particle embedded in an image bearing member shields exposure light by setting a particle diameter of the conductive fine particle to be more than 0.1 μm, thereby realizing an excellent image recording.
Furthermore, JP 2001-235891 A discloses a technique strictly controlling a particle size distribution of a developer containing a conductive fine particle, thereby improving not only direct injection chargeability but also image characteristics.
On the other hand, a number of methods of adding a conductive fine particle to a toner as an external additive have been proposed. For example, it is widely known that a carbon black (as a conductive fine particle) is used as an external additive to be attached or fixed on the surface of a toner for the purpose of providing the toner with conductivity or suppressing excessive charge of the toner to obtain uniform triboelectric charge distribution. Also, JP 60-069660 A discloses that a conductive fine particle such as tin oxide, zinc oxide or titanium oxide is externally added to a magnetic toner having high resistivity. Furthermore, JP 56-142540 A proposes a toner with which satisfactory developing property and transferring property are attained, the toner being prepared by adding a conductive magnetic powder such as iron oxide, iron powder, or ferrite to a magnetic toner having high resistivity so that the conductive magnetic powder accelerates a charge induction to the magnetic toner. In addition, JP 02-120865 A discloses that various conductive fine particles including graphite, magnetite, a conductive polypyrrole powder, and a conductive polyaniline powder are added to a toner.
Various techniques adding two kinds of external additives have also been disclosed. JP 02-45188 A and JP 2,893,147 B propose a technique for improving endurance stability and flowability of a toner by adding two kinds of external additives respectively having a different particle diameter. However, sufficiently satisfactory solutions have not yet been made against problems with regard to durability such as deterioration of a toner due to long-term use, or deterioration of the surface of a toner carrier or abrasion of an image bearing member in a cleaning coinciding with developing or a cleanerless process employing a contact developing system.
Furthermore, although JP 10-307456 A, JP 10-307421 A, JP 10-307455 A, JP 10-307457 A, and JP 10-307458 A somewhat describe a preferred particle diameter of a conductive fine particle, no description has been made with regard to a particle size distribution or components of the conductive fine particle. Therefore, further improvement is required for obtaining stable performance at any time.
With regard to JP 2001-235891 A, sufficiently satisfactory performance is not always obtained. In other words, it is necessary to review a material at an angle different from before. Thus, an improvement is necessary.
The above proposals are based on the premise that the surface of a charging member is not contaminated. However, in view of the fact that it is difficult to avoid contamination of a charging member due to various pattern printing or stop of a machine by unexpected abnormal operation, it is necessary to propose a direct injection-charging system having durability against some degree of contamination of a charging member as soon as possible.
In contrast, JP 11-149205 A proposes an image recording apparatus maintaining satisfactory chargeability by applying, at the time of non-image recording, an alternating voltage to a charging member, which is contaminated by a transfer residual toner at the time of recording an image, to divert contaminated toner to the surface of a photosensitive member.
However, this technique is also based on the concept of suppressing a level of contamination on the surface of a charging member due to a transfer residual toner thereby maintaining satisfactory chargeability. Therefore, in the case where the surface of the charging member is contaminated in unusual use conditions, image quality is largely deteriorated and in a worse case, a printer itself may break down.
Furthermore, when a conductive fine particle as an external additive is added to a toner to improve image characteristics, the additive is selected mainly based on an average particle diameter in many cases. In consideration of interaction between toner particles and those conductive fine particles, it is easily supposed that density of contact point therebetween have significant meaning and therefore largely affect image characteristics similarly to the case of a direct injection-charging system. However, there are very few cases studying this point.